Many types of cars, carts, vehicles and trolleys are supported on bogies or trucks that are designed for engagement with and travel on non-featured rails. A subset of such vehicles constrained to travel on rails includes those engineered for travel on a single rail. The latter are commonly referred to as monorail vehicles. The design and manner of engagement between carriages or bogies of monorail vehicles and the non-featured rail or monorail presents a number of challenges specific to these vehicles.
First, the six degrees of freedom of a vehicle traveling on a monorail must be constrained. Traditionally, these degrees of freedom include the three linear degrees of freedom, namely: longitudinal translation along the rail, lateral translation and vertical translation. There are also the three rotations, namely: rotation about the longitudinal direction (roll), rotation about the lateral direction (pitch), and rotation about the vertical direction (yaw).
Typically, translation along the longitudinal direction (along the rail) is controlled by traction systems of the monorail and therefore does not need to be controlled by the suspension system or bogie. Lateral translation is usually constrained with wheels located on either side of the monorail. Vertical translation is often controlled with wheels located on the top and/or on the bottom surfaces of the monorail. Yaw may be controlled with two wheels that resist lateral translation and are spaced by a certain distance along the longitudinal direction. Similarly, pitch may be controlled with two wheels that are also spaced longitudinally and resist vertical translation.
Roll, the rotation about the longitudinal direction or about the rail is more challenging to constrain. The prior art teaches a number of approaches to limit roll and control roll attitude. These teachings typically fall into one of two general approaches or a combination thereof.
According to the first approach, systems deploy rails with features spread far apart and designed to interface with the bogie. Separately, or in combination, bogie-restraining provisions can be provided to control the roll or maintain a certain roll attitude. In addition, the wheels including traction wheels, support wheels, guide wheels or idler wheels belonging to the bogies and their assemblies may have rims or other structures to help arrest roll. Furthermore, the placement of the center of gravity of the monorail vehicle is used to aid in constraining roll. There are a number of exemplary teachings that fall within this first approach.
For example, U.S. Pat. No. 3,935,822 to Kaufmann teaches a monorail trolley designed to travel on a monorail and having a truck in which the center of gravity of both the loaded and empty trolley truck is displaced with respect to the points of contact between the rail and the supporting wheel and the counter-wheel to cause both wheels to engaged firmly and adhere to the rail. Kaufmann's design accommodates rapid and easy placement of the truck on the monorail and permits the trolley to move up and down grades. However, Kaufman's monorail trolley does not teach to control forces on lateral wheels to control the roll axis and roll attitude and it does not support accurate trolley localization on a non-featured rail. Furthermore, this design is not appropriate for rail that has have long unsupported spans that place restrictions on minimum torsional stiffness, minimum lateral bending stiffness, minimum vertical bending stiffness and maximum material stress.
U.S. Pat. Nos. 3,985,081; 7,341,004; 7,380,507 and U.S. Published Application 2006/0213387 all to Sullivan also teach a rail transportation system and methods in which vehicles on tracks have a center of gravity outside the contact surfaces between the motorized and counterbalance wheels. Because the center of gravity acts outside of the surfaces of contact between the transport unit and the track, the unit will be stable and a sufficiently high force will be generated between the drive wheels and the track web to assure adequate traction over the entire transportation system. Sullivan further suggests that the unit should resist “sway” and “roll” caused by dynamic loading introduced by movement of the units over the track.
However, Sullivan's solutions require at least one beam extending between the guide ways for absorbing torsional forces caused by the composite centers of gravity of the vehicles being offset from the tracks. In fact, a transportation system as taught by Sullivan incurs high torsional forces that would not be appropriate in situations deploying rails having substantially varying profiles (e.g., low-grade stock rails whose cross-sections exhibit substantial profile variation) and rails that contemporaneously have long unsupported spans that place restrictions on minimum torsional stiffness, minimum bending stiffness and maximum material stress.
Further teachings are provided in U.S. Pat. No. 7,823,512 to Timan. Timan's monorail car travels on a monorail track of uniform cross-section and includes guide wheels, load bearing wheels and stabilizing wheels to provide for good travel. Again, although Timan's solutions use uniform cross-section rails and address the roll of the monorail bogie, they are not appropriate for rails whose cross-sections exhibit substantial profile variation and require a vehicle with a multitude of mechanisms for controlling the monorail bogie with respect to the rail.
Still further notable teachings that fall into the first approach are found in U.S. Pat. No. 4,000,702 to Mackintosh; U.S. Pat. No. 6,446,560 to Slocum. In contrast to these solutions, the second general approach involves the use of large springs and/or hydraulic systems to clamp the rail. One advantage of these approaches is the expanded ability to use non-featured rails that are typically more readily available and lower cost. Some systems that deploy springs and/or hydraulics as well as other related solutions are described in U.S. Pat. No. 3,198,139 to Dark; U.S. Pat. No. 3,319,581 to Churchman et al.; U.S. Pat. No. 3,890,904 to Edwards and U.S. Pat. No. 6,523,481 to Hara et al.
Unfortunately, deployment of large opposing springs to clamp the rail is undesirable in many applications. Such mechanisms involve many parts, are unreliable and contribute to vehicle cost and mass.
Further, in the case in which the apparatus must use an unsupported guide rail that is as small and inexpensive as possible and the vehicle of the apparatus must be accurately located, the prior art does not produce a satisfactory solution. Such an inexpensive guide rail is necessarily small, to minimize material use, and exhibits substantial profile variation, to allow for loose manufacturing processes. Further, as the rail is unsupported over long lengths, such a rail would be additionally constrained by limitations on minimum torsional stiffness, minimum lateral bending stiffness, minimum vertical bending stiffness and maximum material stress. These additional requirements mean that the featured cross-sections as taught in the first general approach in the prior art are not viable for unsupported spans. A vehicle would therefore have to interface with a rail without the multiple features to which a vehicle could interface as shown in the prior art. Thus, the prior art struggles to deliver accurate location of a vehicle under these constraints.
For example, in order to locate a point 200 mm away from the rail to within 2 mm, a typical vehicle attached to a rail of a maximum of 100 mm height would require opposing springs on the order of 400 N/mm. Further, on a rail with loose manufacturing tolerances, one would expect variation in thickness of +/−2 mm. To guarantee contact with the rail, a vehicle on such a rail would require springs installed at a nominal deflection of 2 mm, which would translate to an initial preload of 800 N on each wheel. A high preload creates high rolling resistance, increases wheel wear, and increases the amount of deflection seen by the wheel, making this solution undesirable. In other words, a suspension system compatible with low-cost rail using opposing springs would either inaccurately locate to the rail or require excessive preloads to ensure contact during vehicle travel.
Thus, prior art approaches exhibit many limitations that render them inappropriate for controlling roll in monorail vehicles that are deployed on low-cost, low-quality, non-featured stock rails with substantially varying profiles and requiring long unsupported spans.